In general terms, furnaces such as those commonly installed to heat homes operate by drawing air from the living area through the furnace, heating the air, and returning it to the living area. In some heating systems, the air drawn from the living area is also used for combustion before being vented to the outside. Other systems draw combustion air from the outside. Duct systems are generally used to move the air, both heated and unheated, from one location to another. A blower forces the air drawn from the living area for heating through at least one heat exchanger. Usually, more than one heat exchanger is utilized. The heat exchangers are heated by combustion product gases generated by burners in a burner assembly, with each burner aligned with the inlet to one heat exchanger. The burners allow combustion of the gas and direct heated flue gas into the heat exchangers.
The typical heat exchanger includes cells with a channel or pass formed in each cell to direct the flow of flue gas produced by combustion and can be formed from sheet steel ("clamshell" exchangers) or steel tubing ("tubular" exchangers). These cells are typically positioned side by side in a parallel manner and include a predetermined spacing to allow the blower air to flow around the cells. The blower air is thus heated as it circulates over the cells.
When the flue gasses exit the heat exchangers, they are gathered in a collector box. The flue gases are typically withdrawn from the collector box by a draft inducer and ducted to the outside via a flue vent.
Each burner-heat exchanger pair are referred to as a "cell." The larger the furnace, the more cells it contains. Small furnaces typically contain a minimum of two cells. Draft inducers are sized to optimize combustion efficiency. A draft inducer sized for a two-cell furnace does not optimize combustion efficiency for a three-cell furnace, and vice versa. That is, a draft inducer sized for a three-cell furnace is too strong for a two-cell furnace and decreases the efficiency of the furnace. In a product line that includes two-cell through seven-cell furnaces, six different size draft inducers are needed. A need therefore exists to reduce the number of different size draft inducers required for the product line.